Notice in the receptive field on the left that although a small part of the inhibitory surround lies in the white center a small part of it also lies in the darkest part of the figure. With the receptive field placed as it is one might expect that the size of the surround inhibitory response would be about the same as the size of the center excitatory response.Now look at the receptive field on the right. Clearly more of the inhibitory surround lies in either the same gray as the center or in darker areas. Only a small portion lies in a brighter area than the excitatory center.
Consequently, one would expect that the excitatory center would have a larger response on the diagonal than for a receptive field not on a diagonal. Hence there appears a brightness enhancement on the diagonals of this diagram. You, of course, noticed that there is no brightness on the diagonals in this figure. I show why in the next diagram.
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